I have come up with a few ways of keeping someone tied or confined.

The first is to use a baby monitor. That way they can say their safeword and be heard and still be left alone in the room.

The other way is to use tamper tape, a type of tape that tears easily and cant be simply taken off and put back. It could for example be placed on a cage door or on a code padlock. The sub is perfectly able to escape but if they escaped and then put themselves back in their bonds their dom would know.

Third way is similar to the second. Simply put a key in a signed envelope that has to be torn in order to open it. Works about the same as the second method.

Any thoughts? Any more ways to leave someone in captivity in a safe way?